The present invention relates, in general, to a method for producing hardened sheet metal sections and in particular for producing hardened sheet metal sections from a flat sheet metal blank or from a sheet metal blank that has been preformed.
Methods for hardening sheet metal sections from a blank are known in the prior art. For example, DE 24 52 486 A1 discloses a press hardening method, wherein a sheet metal blank from hardenable steel is heated to a hardening temperature and then hotformed and subsequently hardened while the sheet metal section remains within the press tool. As part of the hardening process, the sheet metal section remains clamped within the tool while it is being cooled during a cooling step, thereby ensuring a product of precise measurements and narrow tolerances.
Carrying out the steps of hotforming and hardening the sheet metal section while still in the press tool is advantageous since the combination of forming and hardening the section within the tool is generally very efficient with respect to time and cost.
Furthermore, WO 99/07492 discloses a modification of the afore-described press hardening method, wherein the rim area of the pre-made holes in the sheet metal sections are bent in such a manner that a collar is created. Collaring of the pre-made holes is carried out in the tool but prior to hardening. These holes in the sheet metal serve as lead through holes for screws or other fasteners.
Such collared holes can also be utilized to properly position sheet metal sections in the manufacturing operations described herein. In the respective terminology, such collared holes are also known as collared holes. They find application in particular in the motor vehicle industry, where collared holes are utilized in order to reinforce bores or passages present in car building parts, which are used in car manufacturing such as, for example, in transversal bars. Collared holes of this type are designed so that they can help prevent, for example, that a car building part will fail by pre-maturely buckling when the car is involved in car crash.
During production of flat or preformed sheet metal blanks, it can happen that the collars of the holes which extend from the horizontal plane of the blank get damaged when the blank is transported on a conveyor and the collars of the holes are supporting the blank. In addition, the collars can get caught in the conveyor, when the conveyor is, for example, composed of rollers. If the collars are becoming either deformed or caught while the blank is being transported, the production line is interrupted which leads to inefficient production.
One aspect of the present invention is to improve on the prior art and to provide an efficient method for producing hardened sheet metal sections with collared holes in such a manner so as to obviate shortcomings in the production of prior art sheet metal sections.